Chapter 14
Autonomic Nervous System
(ANS)
Due @ the final
• Labs 21, 22
• Extra credit papers
– Remember to mention which of the papers
you read in your write up
Autonomic Nervous System
• Regulates the visceral response of motor
neurons
– Innervate smooth and cardiac muscle and glands
– Make adjustments to ensure optimal support for body
activities
– Operate via subconscious control
– Have viscera as most of their effectors
• Visceral motor neurons pathways include
– Preganglionic neuron (“to” the ganglion)
– Postganglionic neuron (“from the ganglion)
– Synapse at the ganglion
Somatic versus Autonomic
ANS differs from somatic
• The effectors of the ANS are cardiac muscle,
smooth muscle, and glands (not skeletal)
• Axons of the ANS are a two-neuron chain
– The preganglionic (first) neuron has a lightly
myelinated axon
– The ganglionic (second) neuron extends to an effector
organ
• Preganglionic fibers release ACh
• Postganglionic fibers release norepinephrine or
ACh and the effect is either stimulatory or
inhibitory
Two divisions of the ANS
• Sympathetic
• Parasympathetic
Activity of both is overseen by the
hypothalamus
ANS: Sympathetic Division
• Also called the “thoracolumbar division”
and the “fight or flight” response
• Involves E activities – exercise, excitement,
emergency, and embarrassment
• Preganglionic fibers arise from T1-L2
• Preganglionic fibers are short,
postganglionic are long
• All synapses are close to spinal cord
Responses to Increased
Sympathetic Activity
1.
2.
3.
4.
5.
Heightened mental alertness
Increased metabolic rate
Reduced digestive and urinary functions
Energy reserves activated
Increased respiratory rate and
respiratory passageways dilate
6. Increased heart rate and blood pressure
7. Sweat glands activated
Sympathetic Division
Sympathetic NS
• Three parts
– Sympathetic chain ganglia
– Collateral ganglia
– Adrenal medulla
Sympathetic Chain Ganglia
Communicating Rami
• White Rami – preganglionic fibers
• Gray Rami – postganglionic fibers
Adrenal Medulla
Adrenal Medulla
• Preganglionic fibers entering adrenal
gland proceed to center (adrenal medulla)
• Modified sympathetic ganglion
• Preganglionic fibers synapse on
neuroendocrine cells
• Specialized neurons secrete
neurotransmitters into the bloodstream
that act as hormones (norepinepherine,
epinepherine)
Sympathetic
Innervation
Sympathetic Neurons
• Preganglionic release ACh, always
stimulates the (post)ganglionic neuron
• Postganglionic release NE, (except in
skeletal muscles and sweat glands, which
release ACh)
• Remember that the response is dependent
on the receptor present (NE can be
excitatory or inhibitory)
– Example: lung bronchi smooth muscle is
inhibited by NE, opening op bronchi
Postganglionic ACh
• Skeletal muscles are innervated by ACh
postganglionic fibers of the sympathetic division
(instead of NE)
• Important because ACh causes dilation of these
blood vessels, whereas NE causes constriction
of the blood vessels in the abdominopelvic cavity
• Thus widespead sympathetic activation leads to
a redistribution of blood away from skin and
viscera and into skeletal muscles
 This allows you to run away while holding off on
your digestion until later
Norepinephrine
• Neurons that release NE are called
adrenergic
• Two types of receptors bind NE
– Both are indirect through G proteins
• Tends to cause vasoconstriction
• Much longer lasting and more modualtory
than ACh
ANS: Parasympathetic Division
• Also known as the “rest and repose”
division or the “craniosacral division”
• Preganglionic neurons arise from cranial
nerves 3, 7, 9, 10 and sacral segments
S2-S4 and are LONG
• Postganglionic neurons are short and
arise close to the target organ (often in the
wall of the organ itself)
Parasympathetic
Vagus Nerve (X)
• Provides 75% of all parasympathetic
outflow
• Mr. parasympathetic
Preganglionic fibers in the
sacral segments
• Do not join the ventral roots
• Form the pelvic nerves
Parasympathetic Nervous
System
Parasympathetic NS
• Release ACh at both the pre- and
postganglionic neurons (so ALL
parasympathetic neurons use ACh)
• Remember ACh binds to muscarinic and
nicotinic receptors, so response will
depend on type or receptor that is present
Parasympathetic Activation
• Centers on relaxation, food processing,
and energy absorption
• Localized effects, last a few seconds at
most because ACh is short lived in the
synapse. Why?
Some Effects of
Parasympathetic Activation
1. Constriction of pupils:
2. Secretion by digestive gland
3. Changes in blood flow and glandular activity
associated with sexual arousal
5. Increases smooth muscle activity along
digestive tract
6. Defecation
7. Contraction of urinary bladder
8. Constriction of respiratory passageways
9. Reduction in heart rate and force of contraction
Comparing Sympathetic and
Parasympathetic Divisions
• Sympathetic:
– widespread impact
– reaches organs and tissues throughout body
• Parasympathetic:
– innervates only specific visceral structures
– effects are shorter lived (why?)
Anatomy of ANS
Division
Origin of Fibers
Length of Fibers
Location of
Ganglia
Sympathetic
Thoracolumbar
region of the spinal
cord
Short preganglionic Close to the
and long
spinal cord
postganglionic
Parasympathetic
Brain and sacral
spinal cord
Long preganglionic
and short
postganglionic
In the visceral
effector organs
Comparison of Sympathetic NS
and Parasympathetic NS
Effects of Drugs
• Atropine – blocks parasympathetic effects
• Neostigmine – inhibits
acetylcholinesterase and is used to treat
myasthenia gravis
• Tricyclic antidepressants – prolong the
activity of NE on postsynaptic membranes
• Over-the-counter drugs for colds, allergies,
and nasal congestion – stimulate adrenergic receptors
• Beta-blockers – attach mainly to 1
receptors and reduce heart rate and
prevent arrhythmias
Sympathetic and Parasympathetic
Autonomic Tone
• Autonomic tone refers to constant activity
of the cells of the ANS
• An important aspect of ANS function
because:
– If a nerve is inactive under normal conditions,
it can only increase its activity
– However, if nerve maintains a constant
background level of activity, then it can either
increase or decrease its activity
Single Innervation
Case study: Blood vessels
• Only contacted by sympathethic NS
• NE (released at most vessels): constrictis
– Thus an autonomic tone keeps the vessels
half way consticted/dilated at rest
• Sympathetic ACh released at skeletal
muscles causes dilation
Dual Innervation
• Many vital organs receive instructions from
both sympathetic and parasympathetic
divisions (a notable exception is the blood
vessels)
• 2 divisions commonly have opposing
effects (e.g. speed vs. slow the heart)
Dual Innervation:
The Heart
• Receives dual innervation
• 2 divisions have opposing effects:
– parasympathetic division:
• acetylcholine released by postganglionic fibers
slows heart rate
– sympathetic division:
• NE released by postganglionics accelerates heart
rate
The Heart
• Balance between 2 divisions:
– autonomic tone is present
– releases small amounts of both
neurotransmitters continuously
• Parasympathetic innervation dominates
under resting conditions
• How do we know this?
Autonomic Innervation:
The Heart
• Heart rate controlled precisely through
small adjustments
• Crisis accelerates heart rate by:
– stimulation of sympathetic innervation
– inhibition of parasympathetic innervation
– BOTH together allows for a bigger effect
Summary: Sympathetic and
Parasympathetic Divisions
• Know this:
Table 16-2